I have a Whirlpool mh3184xpq-0 microwave/range fan which was made in 2004. Just recently the fan motor has started humming and when I removed the grate from the fan I noticed that the fan blades are spinning very slowly on their own. The fan runs as normal when I turn it on but when I shut it off it continues to hum and spin.

If I cut the power to it for awhile and then turn the power back on it wont do it for an hour or two but then it will start again.

I'm guessing it's more economical to replace the entire thing rather than repair it but I'm curious what might be wrong with it.

Sounds like a control board problem. Bad capacitor or something in the control and is also not giving it full power to operate.

I would be afraid of an electric fire and would just buy a new one.

Thanks for posting a good model #. With that, I was able to look at the wiring diagram. If appears that you either have a failed triac or hood fan (HF) relay. Both are located on the electronic control. The control is W10233386 but No Longer Available.

For the present, you can simply unplug the blower motor. Ultimately, you are talking replacement.